This invention relates to synchronization of events between multiple marine seismic vessels. A particular application of the invention relates to synchronization of sources and receivers on different vessels.
Referring now to FIG. 1, an example of a multi-vessel marine seismic formation is shown in which a master vessel 100 is moving in parallel with slave vessel 102. Both master vessel 100 and the slave vessel 102 are towing streamers 100a-100b and 102a-102b, respectively. Master vessel 100 also tows sources 100c and 100d, and the slave vessel 102 also tows sources 102c and 102d. As will be understood by those of skill in the art, streamers 100a-100b and 102a-102b include seismic signal receiving sensors (for example, hydrophones), which receive reflections of signals from sources 100c-100d and 102c-102d (for example, aqua pulse guns). As is understood by those of skill in the art, sources 100c-100d are controlled by a source controller (for example, a GCS-90 source controller) located on master vessel 100. Likewise, sources 102c and 102d are controlled by a source controller located on slave vessel 102. Signals received by streamers 100a-100b are recorded onboard master vessel 100 by a recorder (for example, a Syntrak-480 acquisition system). Likewise, signals received by streamers 102a and 102b are recorded by a recorder on slave vessel 102.
In many cases, it is desirable to record reflections of signals initiated by sources 102c and 102d at streamers 100a and 100b, and it is also desirable to record reflections from signals which are generated by sources 100c-100d and received by streamers 102a and 102b. In such situations, synchronization of the recording system on one vessel with the source on another vessel is critical to accurate data acquisition. However, while the recording systems on any given vessel are accurately synchronized with sources on the same vessel, precise coordination of recorders and sources between vessels has proved to be difficult.
An example of the problem occurs when the recording system on the master vessel is set to record seismic reflections from a source on a slave vessel. The master vessel transmits a command to the slave vessel to shoot the slave vessel's seismic source. There is a small, but significant, delay between the instant when the master vessel computer issues the command for the slave vessel source to shoot, and at the instant when the slave vessel actually causes the shot. This delay is caused by the delay inherent in the computers, radio transmission, and receiving links between the vessels.
The instant in time when any source actually fires, and the instant when any particular reflection is received by a streamer, are termed "events" which must be synchronized. Those of skill in the art will also recognize that synchronization among and between other events is also of critical importance in multi-vessel seismic exploration. Examples of such other events include: the instant in time when a particular vessel crosses a particular point on the seabed floor, the instant in time when a seismic source on a particular vessel is initiated, etc.
According to typical systems, a VHF radio link is used to communicate the events between the two vessels, with, for example, a phase locked loop circuit used to detect the events communicated on the radio link. "Blast" and "time break" commands are generated across the radio link at specific instants, based upon the calculated delay, which will, hopefully, cause the recorder to begin recording at about the same instant as the firing of the source. However, such a system requires a constantly operational radio transmission link, and the system also requires regular calibration. Calibration is normally carried out "off-line," the result of which is that timing errors may occur between calibrations that are undetected. Also, a constantly operational radio link can be broken. If broken at a time when a command to shoot the slave vessel's source is transmitted, the command will not be received, the shot will not be made, and the vessels will pass by a spot where data is required.
Such synchronization problems have been recognized in the seismic art in the past. For example, U.S. Pat. No. 4,511,999, issued to Bowden, et al. on Apr. 16, 1985, incorporated herein by reference, discloses a need for a system that provides the capability to synchronize firing of seismic sources at a precise time coinciding with times from a clock in a remote recording system. Bowden's discussion of prior art discloses devices for synchronizing seismic sources which calculate and adjust delay after receiving firing signals. Bowden's "Summary of Invention" discloses a gun shot control for seismic exploration in which a digital clock produces regularly occurring timing pulses, including shot timing pulses. An adjustable digital counter counts clock pulses in response to the occurrence of a shot timing pulse and produces a delayed shot command signal at an adjustable delay time from the shot timing signal.
The problem of synchronization between vessels is also noted in U.S. Pat. No. 4,300,653, issued to Cao, et al. on Nov. 17, 1981, incorporated herein by reference, which discloses an apparatus for controlling a precise time in firing each air gun in an array of air guns employed in marine seismic exploration. The apparatus keeps a record of past delays, and, upon receipt of a fire command, inserts appropriate delays for triggering individual air guns, based on the fire command. It will be noted that the problems discussed above with synchronization are still inherent in both Bowden and Cao; that is, both Bowden and Cao attempt to accurately calculate delay between events.
Another reference discussing synchronization of seismic sources is U.S. Pat. No. 4,047,591, issued to Ward, et al. on Sep. 13, 1977, incorporated herein by reference, which discloses an air gun construction allowing synchronization in timing of multiple air guns relative to one another. Processing circuitry associated with the air gun generates a timing signal allowing for phasing of multiple air guns, similar to the phase locked loop process discussed above.
An alternative prior art device is seen in U.S. Pat. No. 3,985,199, issued to Baird on Oct. 12, 1976 and incorporated herein by reference, which discloses an apparatus for affecting and controlling the firing of energy sources. Baird teaches the use of programmable counters for initiating the firing of different sources and a reference time counter to initiate each of the programmable counters.
U.S. Pat. No. 3,972,019, issued to Bassett on Jul. 27, 1976 and incorporated herein by reference, discloses independently operating timing units located at distant points. The units operate without transmission of timing signals between the units, because means for providing synchronization of the units are used when one unit is connected directly to the other. Bassett recognizes that coded radio systems used for controlling of remote firing systems depend upon the establishment of the same time scale at control and source locations in which fresh synchronization has to be performed for each record, resulting in overall delays which were rarely inside a designated plus or one millisecond standard. Therefore, two timing units are disclosed, one located at a remote receiving station and the other at a source station. Each unit contains an identical high stability temperature-controlled crystal oscillator, whose frequency is divided down to produce an output in intervals. Before surveying is begun (for example, at the start of each day), the two timing units are directly connected to each other for synchronization. Such a system cannot be monitored for synchronization error as the day progresses.
U.S. Pat. No. 3,902,161, issued to Kiowski, et al., on Aug. 26, 1975 and incorporated herein by reference, discloses a remote triggering device for one or more receiver stations using phase encoding of a series of binary numbers on a tone, and transmitting this series to receiving stations via radio transmission links. Synchronization is achieved by a phase locked loop, and thus has the same inherent problems discussed above.
U.S. Pat. No. 3,733,584, issued to Pelton, et al., on May 15, 1973 and incorporated herein by reference, discloses a system for remotely controlling and activating a seismic vibrator. Two clocks are used for the receiver and vibrator, which are synchronized at one time by physical connection (similar to Bassett). Pelton notes that when radio noise is high, or the distance of transmission is great, transmission of coded time signals may not be satisfactory, so Pelton provides for a system of operation in which no signal contact of any kind is required between the recording station and the vibrator. Thus, like Bassett, Pelton does not allow for exchange of information between remote units, nor does Pelton provide for re-synchronization to occur during surveying sessions.
It is apparent from the above references, that there is a need for dealing with the problems associated in delay between triggering signals and the actual occurrence of events in seismic exploration. It is also apparent from the above references that no system currently exists which allows for radio communication between master and slave vessels that is tolerant of radio noise, while still providing constant calibration and accurate synchronization between events on the master and slave vessel.